Mendonca, Paulo E. M. F.Marchiolli, Marcelo A.Galetti, Diogenes [UNESP]2015-03-182015-03-182014-12-01Annals Of Physics. San Diego: Academic Press Inc Elsevier Science, v. 351, p. 79-103, 2014.0003-4916http://hdl.handle.net/11449/116411We demonstrate that for every two-qubit state there is a X-counterpart, i.e., a corresponding two-qubit X-state of same spectrum and entanglement, as measured by concurrence, negativity or relative entropy of entanglement. By parametrizing the set of two-qubit X-states and a family of unitary transformations that preserve the sparse structure of a two-qubit X-state density matrix, we obtain the parametric form of a unitary transformation that converts arbitrary two-qubit states into their X-counterparts. Moreover, we provide a semi-analytic prescription on how to set the parameters of this unitary transformation in order to preserve concurrence or negativity. We also explicitly construct a set of X-state density matrices, parametrized by their purity and concurrence, whose elements are in one-to-one correspondence with the points of the concurrence versus purity (CP) diagram for generic two-qubit states. (C) 2014 Elsevier Inc. All rights reserved.79-103engEntanglementConcurrenceNegativityRelative entropy of entanglementX-stateArtigo10.1016/j.aop.2014.08.017WOS:000346451900007Acesso restrito